?? T cell receptor (TCR) recognition of peptide antigens presented by major histocompatibility complex proteins (pMHC) is a cornerstone of cellular immunity, defining specificity and initiating the signaling that leads to T cell-mediated immune responses. Although data for TCR-pMHC interactions has increased significantly over the past decade, we still have a poor understanding of how T cells and TCRs achieve their extraordinary recognition properties. Nonetheless, there continues to be rapid growth in the use of TCRs and T cell epitopes for immunotherapy, particularly for cancer. While there have been immunotherapy successes, there have also been complications and setbacks. It is widely accepted that an improved understanding of TCR/T cell recognition is needed for such therapies to reach their potential. This proposal describes an ambitious, tightly integrated study that blends protein biophysics, structural biology, computation, and immunology to understand TCR recognition, signaling, and immune modulation. It also describes the development of general strategies for designing and improving immune-based therapeutics by incorporating advances in TCR recognition into efforts in computational design and modeling, and vice versa. Priorities include 1) determining how TCR-pMHC binding is shaped by evolutionary and non-evolutionary forces; 2) understanding the varied mechanisms underlying specificity and cross-reactivity in T cell recognition; 3) determining the role of molecular motion in TCR triggering, including the structural and dynamical properties of intact TCR/CD3 complexes; 4) learning how to design TCRs with improved molecular recognition properties, emphasizing antigen specificity and the ability to carefully modulate rather than simply enhance affinity; 5) developing computational techniques for high throughput modeling of TCR cross-reactivity and the structural/dynamical properties of neo-antigens in MHC proteins; and 6) learning how peptides alter MHC properties beyond those ascertainable from static structures and how this modulation impacts cellular immunity.